Even though cosmology
doesn't have that much to do with information It certainly does have
to do with revolution and phase transitions, in fact phase transitions
in both the literal and the figurative sense of the word.

[ALAN GUTH:] Even though cosmology doesn't have that much to do with
information, it certainly has a lot to do with revolution and phase
transitions. In fact, it is connected to phase transitions in both the
literal and the figurative sense of the phrase.

It's
often said  and I believe this saying was started by the late
David Schramm  that today we are in a golden age of cosmology.
That's really true. Cosmology at this present time is undergoing a
transition from being a bunch of speculations to being a genuine branch
of hard science, where theories can be developed and tested against
precise observations. One of the most interesting areas of this is
the prediction of the fluctuations, the non-uniformities, in the cosmic
background radiation, an area that I've been heavily involved in.
We think of this radiation as being the afterglow of the heat of the
Big Bang. One of the remarkable features of the radiation is that
it's uniform in all directions, to an accuracy of about one part in
a hundred thousand, after you subtract the term that's related to
the motion of the earth through the background radiation.

I've
been heavily involved in a theory called the inflationary universe,
which seems to be our best explanation for this uniformity. The uniformity
is hard to understand. You might think initially that maybe the uniformity
could be explained by the same principles of physics that cause a
hot slice of pizza to get cold when you take it out of the oven; things
tend to come to a uniform temperature. But once the equations of cosmology
were worked out, so that one could calculate how fast the universe
was expanding at any given time, then physicists were able to calculate
how much time there was for this uniformity to set in.

They
found that, in order for the universe to have become uniform fast
enough to account for the uniformity that we see in the cosmic background
radiation, information would have to have been transferred at approximately
a hundred times the speed of light. But according to all our theories
of physics, nothing can travel faster than light, so there's no way
that this could have happened. So the classical version of the Big
Bang theory had to simply start out by assuming that the universe
was homogeneous  completely uniform  from the very beginning.

The
inflationary universe theory is an add-on to the standard Big Bang
theory, and basically what it adds on is a description of what drove
the universe into expansion in the first place. In the classic version
of the Big Bang theory, that expansion was put in as part of the initial
assumptions, so there's no explanation for it whatever. The classical
Big Bang theory was never really a theory of a bang; it was really
a theory about the aftermath of a bang. Inflation provides a possible
answer to the question of what made the universe bang, and now it
looks like it's almost certainly the right answer.

Inflationary
theory takes advantage of results from modern particle physics, which
predicts that at very high energies there should exist peculiar kinds
of substances which actually turn gravity on its head and produce
repulsive gravitational forces. The inflationary explanation is the
idea that the early universe contains at least a patch of this peculiar
substance. It turns out that all you need is a patch; it can actually
be more than a billion times smaller than a proton. But once such
a patch exists, its own gravitational repulsion causes it to grow,
rapidly becoming large enough to encompass the entire observed universe.

The
inflationary theory gives a simple explanation for the uniformity
of the observed universe, because in the inflationary model the universe
starts out incredibly tiny. There was plenty of time for such a tiny
region to reach a uniform temperature and uniform density, by the
same mechanisms through which the air in a room reaches a uniform
density throughout the room. And if you isolated a room and let it
sit long enough, it will reach a uniform temperature as well. For
the tiny universe with which the inflationary model begins, there
is enough time in the early history of the universe for these mechanisms
to work, causing the universe to become almost perfectly uniform.
Then inflation takes over and magnifies this tiny region to become
large enough to encompass the entire universe, maintaining this uniformity
as the expansion takes place.

For
a while, when the theory was first developed, we were very worried
that we would get too much uniformity. One of the amazing features
of the universe is how uniform it is, but it's still by no means completely
uniform. We have galaxies, and stars and clusters and all kinds of
complicated structure in the universe that needs to be explained.
If the universe started out completely uniform, it would just remain
completely uniform, as there would be nothing to cause matter to collect
here or there or any particular place.

I believe
Stephen Hawking was the first person to suggest what we now think
is the answer to this riddle. He pointed out  although his first
calculations were inaccurate  that quantum effects could come
to our rescue. The real world is not described by classical physics,
and even though this was very "high-brow" physics, we were in fact
describing things completely classically, with deterministic equations.
The real world, according to what we understand about physics, is
described quantum-mechanically, which means, deep down, that everything
has to be described in terms of probabilities.

The
"classical" world that we perceive, in which every object has a definite
position and moves in a deterministic way, is really just the average
of the different possibilities that the full quantum theory would
predict. If you apply that notion here, it is at least qualitatively
clear from the beginning that it gets us in the direction that we
want to go. It means that the uniform density, which our classical
equations were predicting, would really be just the average of the
quantum mechanical densities, which would have a range of values which
could differ from one place to another. The quantum mechanical uncertainly
would make the density of the early universe a little bit higher in
some places, and in other places it would be a little bit lower.

So,
at the end of inflation, we expect to have ripples on top of an almost
uniform density of matter. It's possible to actually calculate these
ripples. I should confess that we don't yet know enough about the
particle physics to actually predict the amplitude of these ripples,
the intensity of the ripples, but what we can calculate is the way
in which the intensity depends on the wavelength of the ripples. That
is, there are ripples of all sizes, and you can measure the intensity
of ripples of different sizes. And you can discuss what we call the
spectrum  we use that word exactly the way it's used to describe
sound waves. When we talk about the spectrum of a sound wave, we're
talking about how the intensity varies with the different wavelengths
that make up that sound wave.

We do
exactly the same thing in the early universe, and talk about how the
intensity of these ripples in the mass density of the early universe
varied with the wavelengths of the different ripples that we're looking
at. Today we can see those ripples in the cosmic background radiation.
The fact that we can see them at all is an absolutely fantastic success
of modern technology. When we were first making these predictions
back in 1982, at that time astronomers had just barely been able to
see the effect of the earth's motion through the cosmic background
radiation, which is an effect of about one part in a thousand. The
ripples that I'm talking about are only one part in a hundred thousand
 just one percent of the intensity of the most subtle effect
that it had been possible to observe at the time we were first doing
these calculations.

I never
believed that we would ever actually see these ripples. It just seemed
too far fetched that astronomers would get to be a hundred times better
at measuring these things than they were at the time. But, to my astonishment
and delight, in 1992 these ripples were first detected by a satellite
called COBE, the Cosmic Background Explorer, and now we have far better
measurements than COBE, which had an angular resolution of about 7
degrees. This meant that you could only see the longest wavelength
ripples. Now we have measurements that go down to a fraction of a
degree, and we're getting very precise measurements now of how the
intensity varies with wavelength, with marvelous success.

About
a year and a half ago, there was a spectacular set of announcements
from experiments called BOOMERANG and MAXIMA, both balloon-based experiments,
which gave very strong evidence that the universe is geometrically
flat, which is just what inflation predicts. (By flat I don't mean
two-dimensional; I just mean that the three-dimensional space of the
universe in not curved, as it could have been, according to general
relativity.) You can actually see the curvature of space in the way
that the pattern of ripples has been affected by the evolution of
the universe. A year and a half ago, however, there was an important
discrepancy that people worried about; and no one was sure how big
a deal to make out of it. The spectrum they were measuring was a graph
that had, in principle, several peaks. These peaks had to do with
successive oscillations of the density waves in the early universe,
and a phenomenon called resonance that makes some wavelengths more
intense than others. The measurements showed the first peak beautifully,
exactly where we expected it to be, with just the shape that was expected.
But we couldn't actually see the second peak.

In order
to fit the data with the theories, people had to assume that there
were about ten times as many protons in the universe as we actually
thought, because the extra protons would lead to a friction effect
that could make the second peak disappear. Of course every experiment
has some uncertainty  if an experiment is performed many times,
the results will not be exactly the same each time. So we could imagine
that the second peak was not seen purely because of bad luck. However,
the probability that the peak could be so invisible, if the universe
contained the density of protons that is indicated by other measurements,
was down to about the one percent level. So, it was a very serious-looking
discrepancy between what was observed and what was expected. All this
changed dramatically for the better about 3 or 4 months ago, with
the next set of announcements with more precise measurements. Now
the second peak is not only visible, but it has exactly the height
that was expected, and everything about the data now fits beautifully
with the theoretical predictions. Too good, really. I'm sure it will
get worse before it continues to get better, given the difficulties
in making these kinds of measurements. But we have a beautiful picture
now which seems to be confirming the inflationary theory of the early
universe.

Our
current picture of the universe has a new twist, however, which was
discovered two or three years ago. To make things fit, to match the
observations, which are now getting very clear, we have to assume
that there's a new component of energy in the universe that we didn't
know existed before. This new component is usually referred to as
"dark energy." As the name clearly suggests, we still don't know exactly
what this new component is. It's a component of energy which in fact
is very much like the repulsive gravity matter I talked about earlier
 the material that drives the inflation in the early universe.
It appears that, in fact, today the universe is filled with a similar
kind of matter. The antigravity effect is much weaker than the effect
that I was talking about in the early universe, but the universe today
appears very definitely to be starting to accelerate again under the
influence of this so-called dark energy.

Although
I'm trying to advertise that we've understood a lot, and we have,
there are still many uncertainties. In particular, we still don't
know what most of the universe is made out of. There's the dark energy,
which seems to comprise in fact about 60% of the total mass/energy
of the universe. We don't know what it is. It could in fact be the
energy of the vacuum itself, but we don't know that for a fact. In
addition, there's what we call dark matter, which is another 30%,
or maybe almost 40%, of the total matter in the universe; we don't
know what that is, either. The difference between the two is that
the dark energy causes repulsive gravity and is smoothly distributed;
the dark matter behaves like ordinary matter in terms of its gravitational
properties  it's attractive and it clusters; but we don't know
what it's made of. The stuff we do know about  protons, neutrons,
ordinary atoms and molecules  appear to comprise only about
5% of the mass of the universe.

The
moral of the story is we have a great deal to learn. At the same time,
the theories that we have developed so far seem to be working almost
shockingly well.